1. Field of the Invention
The present invention relates to nanotechnology, nanotubes and nanowires and processes for the manufacture of nanosize technology.
2. Background of the Art
Nanotubes and nanowires of various materials have become the subjects of intense, global research efforts in recent years. Fabrication of NTs and NWs made from carbon, nitrides and oxides, metals (e.g., Published US Patent Application 20060289351) and mixtures thereof (e.g., Published US Application 20070057415) have been reported. The interesting combination of electrical and mechanical properties of these NT and NW structures has raised possibilities of revolutionizing fields ranging from computing, optics, field emitter devices, sensors, electrodes, solar cells, high strength composites, hydrogen storage and many other applications.
To date, the most widely used process used to grow NT and NW is a Chemical Vapor Deposition (CVD) process. In CVD process, a substrate, often predeposited with catalytic particles of transitional metals, is heated up to high temperature in presence of high pressure (up to 20 Torr) of feedstocks (such as CO or a hydrocarbon in case of growing carbon based NT and NW). One type of CVD process relies solely on heating of the substrate to promote catalytic breakdown of the feedstock at or around the particles and subsequent formation of NTs or NWs at the particles. This type of CVD process is known as thermal CVD. In another type of CVD process, a strong electric field is applied during the growth process. Introduction of the electrical field generates plasma of the feedstock gas around the substrate. This type of CVD process is called Plasma Enhanced Chemical Vapor Deposition (PECVD). In PECVD, growth of NTs and NWs is facilitated by presence of more reactive gases created from dissociation and ionization of the feedstock gas within the plasma. Moreover, presence of electric field forces NTs and NWs to grow in the direction of the electric field and thus achieving superior alignment of the NTs and NWs.
Although PECVD and thermal CVD have clearly demonstrated to be capable of producing NTs and NWs, there are some fundamental limitations associated with these processes. First, high-pressure requirements of the CVD processes do not allow growth surface of the substrate to be clean to atomic level and kept free of surface contaminants for any appreciable time inside the reactor. This is true for both CVD growth techniques, thus NTs and NWs are grown on top of a surface that is far from atomically clean. High-pressure conditions of the CVD reactors also contribute to increased contamination of exterior of NTs and NWs during and after the growth. Second, the high-pressure requirements effectively eliminate usage of many, if not all, powerful in-situ deposition and analysis tool that requires low-pressure environment to operate. Thus, in-situ processing and monitoring are difficult to carry out in the two CVD processes. In PECVD, the chemical reactions at the substrate surface are difficult to control as the surface is directly exposed to the plasma consisting of many species of radicals, neutrals, ions and electrons of different energies. Nanowires can also be [provided with heterostructure by varying crystalline composition along its length as taught in U.S. Pat. No. 6,882,051.
All references cited herein are incorporated herein in their entirety.